1. Field of the Invention
The present invention relates generally to image processing, and more particularly to an apparatus for mixing a playback composite video signal produced by playing a recording medium such as a video disk and a graphics composite video signal produced on the basis of image data read from a memory device.
2. Description of the Related Background Art
Japanese Patent Kokai No. 1-256071 (1989), for example, discloses an apparatus for mixing a playback composite video signal indicative of a moving picture or a still picture produced by playing a recording medium such as a video disk and a graphics composite video signal indicative of a graphics image based on graphics data derived as sub-code signals. In the apparatus, the playback composite video signal and graphics composite video signal are respectively passed through variable resistors and added to each other, wherein the resistance value of each variable resistor is changed in accordance with a manual operation, thus arbitrarily determining a mixing ratio of the playback composite video signal and the graphics composite video signal.
Incidentally, the playback composite video signal conforms to the NTSC standard signals used in the normal television broadcasting, where the horizontal scanning frequency is 15.734 KHz and the number of horizontal scanning lines per frame is 525. On the other hand, while the graphics composite video signal has the same horizontal scanning frequency as the play video signal, the number of scanning lines per frame, determined therefor, is 524 or 526. Since the numbers of the horizontal scanning lines are different by one between the playback composite video signal and the graphics composite video signal, they cannot be vertically synchronized to each other, thereby causing a phenomenon that an image runs on the screen and presenting difficulties in viewing such an image.